netfs: Fix NULL pointer dereference in netfs_unbuffered_write() on retry

In the Linux kernel, the following vulnerability has been resolved:

netfs: Fix NULL pointer dereference in netfs_unbuffered_write() on retry

When a write subrequest is marked NETFS_SREQ_NEED_RETRY, the retry path

in netfs_unbuffered_write() unconditionally calls stream->prepare_write()

without checking if it is NULL.

Filesystems such as 9P do not set the prepare_write operation, so

stream->prepare_write remains NULL. When get_user_pages() fails with

-EFAULT and the subrequest is flagged for retry, this results in a NULL

pointer dereference at fs/netfs/direct_write.c:189.

Fix this by mirroring the pattern already used in write_retry.c: if

stream->prepare_write is NULL, skip renegotiation and directly reissue

the subrequest via netfs_reissue_write(), which handles iterator reset,

IN_PROGRESS flag, stats update and reissue internally.